This invention relates to a video switching circuit which conducts switching between a Y/C separated video signal in which a luminance signal and a chrominance signal are separated, plural composite video signals composed of the luminance signal and the chrominance signal, and a television receiver incorporating this video switching circuit.
Most modern television receivers are designed to display normal television signals received through their own tuner circuits as well as video signals produced by an independent audio-video (AV) apparatus. Television receivers are normally provided with input terminals to which a Y/C separated video signal (hereinafter this is referred to as S-video signal) produced by an independent AV apparatus is connected, in addition to plural input terminals to which plural composite video signals (hereinafter this is referred to as N-video signal) are connected.
The switching between these plural N-video signals connected to the input terminals of a television receiver has been performed by inputting, to the switching circuit, a keying signal such as the one produced by a remote controller.
However, when an S-video signal is connected to the television receiver, the connection is made at a terminal which is common to an N-video signal, and the switching circuit is designed to select the S-video signal preferentially.
The object of this preference is to simplify both the circuit construction and the switching operation of such. Therefore, this enables the connections of both the N-video signal and the S-video signal at a common input terminal, and the consistent display of the S-video signal having substantially better image quality than that of the N-video signal.
In addition to this, the conventional television circuit has been provided with an on-screen display circuit in order to display the type of the selected input video signal on the television screen. The display is then controlled by a micro-computer to which the information identifying the type of input signal is inputted.
A conventional circuit incorporating these features is shown in FIG. 6, wherein 1 is a microcomputer which controls the on-screen display circuit incorporated in the television receiver, 2 is an S-video signal input terminal, 3 is an analog switch IC to select a desired video signal (in this case, among four video signals including the video signals, i.e., AV1, AV2, AV3, and the TV signal), 4 is a circuit block to separate the Y/C video signal components, 5 is an analog switch IC to select either the N- or S-video signal, 6 is a video chrominance IC, 7 is a remote-controller, and 8 is a mixer to produce a signal commanding the analog switch IC 5 to switch to the S-video signal. This signal is produced by mixing a signal showing the presence of an S-video signal at the input terminal and a switching signal produced by the micro-computer 1.
Explaining in more detail the conventional circuit construction shown in FIG. 6, the circuit is equipped with three video input terminals named AV1, AV2, and AV3, respectively. In addition, the S-video signal inputted to the S-video signal input terminal 2 is at a video-signal selecting position which is shared with the input terminal AV2.
The switching between these inputted video signals is performed by means of a micro-computer 1 by which the analog switches IC3 and IC5 are controlled whenever a switching command from the remote-controller 7 is detected by the micro-computer 1.
The switching of the analog switch IC5 in this case is performed by a composite signal comprised of a signal from the micro-computer 1 and a signal from S-video signal input terminal 2. Although the microcomputer 1 may select the video input terminal AV2, it switches to the S-video signal automatically whenever the connection of the S-video signal to the input terminal 2 is detected.
Furthermore, since the signal informing the connection of the S-video signal to the input terminal 2 is fed back to the micro-computer 1, the micro-computer commands display of the type of inputted video signals (i.e. whether it is an N-video signal or an S-video signal) on the screen.
When the input terminal which is shared with the N-video signal and the S-video signal is selected, that is, when an N-video signal and an S-video signal are simultaneously inputted to the common input terminal, the S-video signal is preferentially selected by the video switching circuit incorporated in the conventional television receiver. Therefore, it is then impossible for users to select the N-video signal or the S-video signal by operating the remote-controller key.
However, since various video-software packages are now available to users, users may be dissatisfied by this video signal selection system by which the S-video signal with a higher image quality is preferentially selected ignoring the other N-video signals with lower image quality.
Moreover, since the information relating to the S-video signal selection is inputted to the micro-computer, the on-screen display (OSD) which shows the type of video input, i.e. whether it is an S-video signal or an N-video signal, has to be controlled by the micro-computer whenever the selection is made.